The present invention relates to a wet-processing method for combustion ashes of petroleum fuels, containing ammonium sulfate and a method of utilizing ammonia components recovered by the wet-processing method. More particularly, the present invention relates to a wet-processing method which comprises at least a preparing-step of a slurry of combustion ashes of mixing combustion ashes and water, a double decomposition step for ammonium sulfate, a recovering-step of free ammonia from a gypsum slurry containing free ammonia produced in the double decomposition step, and a gypsum separation step successively, which is excellent in an ammonia stripping efficiency from the gypsum slurry in the ammonia recovery step and in which the clogging in a stripping device is suppressed, as well as a method of utilizing ammonia components with an industrial advantage.
In various kinds of combustion furnaces (combustion devices) using petroleum fuels such as heavy oils and ore emulsions, etc., for example, boilers of oil fired power stations and dust incinerators, ammonia is added in the combustion gases for preventing the corrosion of the combustion furnaces, caused by sulfate gases (SO.sub.3) contained in combustion gases.
Accordingly, the combustion ashes collected and recovered by an electrostatic precipitator disposed to the downstream of a fire flue end thereof contain ammonium sulfate in addition to ashes mainly composed of unburnt carbon and heavy metals (Ni, V, Mg, etc.). For example, the following ingredients are contained in the combustion ashes recovered from a boiler using high sulfur-content heavy oils.
C: 10 to 80 % by weight NH.sub.4 : 0.5 to 20 % by weight SO.sub.4 : 20 to 60 % by weight V: 1 to 5 % by weight Ni: 0.3 to 2 % by weight Fe: 0.3 to 2 % by weight Mg: 0.1 to 8 % by weight SiO.sub.2 : about 0.1 % by weight
As a method of treating the combustion ashes, a lot of methods, referred to as a wet-process, for recovering valuable ingredients such as vanadium, which take a countermeasure for public pollution by a closed system, have been proposed. Specifically, there can be mentioned wet-processes, for example, as disclosed in Japanese Patent Application Laid-Open (KOKAI) Nos. 60-19086 and 60-46930, and Japanese Patent Publication (KOKOKU) Nos. 4-61709 and 5-13718 already proposed.
For example, the process described in Japanese Patent Publication (KOKAI) No. 5-13718 comprises (i) a first step of mixing combustion ashes and water to form a slurry, while optionally adding sulfuric acid to control the pH value to not more than 3, (ii) a second step of separating solids (unburnt carbon, etc.), (iii) a third step of heating the obtained liquid portion to a temperature of not less than 70.degree. C. and oxidizing metals while supplying ammonia and an oxidant to adjust the pH value to 7 to 8, (iv) a fourth step of separating deposits (iron sludges), (v) a fifth step of cooling the obtained liquid portion to a temperature of 40.degree. C., thereby depositing vanadium compounds (ammonium metavanadate), (vi) a sixth step of separating the deposited vanadium compounds, (vii) a seventh a step of adding calcium hydroxide or calcium oxide to the obtained liquid portion to deposit gypsum and metal (nickel and magnesium) hydroxides, and liberating ammonia simultaneously, (viii) an eighth step of recovering ammonia by stripping from free ammonia, and (ix) a ninth step of separating the obtained gypsum.
However, there are drawbacks in the ammonia recovery methods described in the above-mentioned publications, respectively.
Namely, the method disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 60-19086 is a method comprising complicated steps of precipitating gypsum from a slurry before stripping, and passing the resultant supernatant through a distillation column to strip an ammonia gas. Such a method is industrially disadvantageous, in which calcium compounds such as gypsum and magnesium hydroxide tend to be deposited on the inner wall surface of the distillation column and the operation thereof can not be continued for a long time.
Further, the method described in Japanese Patent Application Laid-Open (KOKAI) No. 60-46930 is a method comprising supplying steam from the lower portion of an aerating tank so as to heat to a temperature of not less than 80.degree. C. and supplying air from the lower portion thereof so as to strip ammonia. In this method, the stripping efficiency is poor because the contact between the slurry and air is insufficient, and continuous operation is difficult.
Further, the methods described in Japanese Patent Publication (KOKOKU) Nos. 4-61709 and 5-13718 comprise supplying a slurry containing a large amount of gypsum to a separation column from the upper portion thereof but no concrete example for the separation column is described.
Referring to the recovered ammonia, it is supplied to a fire flue of a boiler or a metal oxidation step for re-utilization. However, since ammonia separated from each of the processes described above is a gasous mixture containing a water content (moisture) evaporated from the slurry and a large amount of air, pipelines of a large diameter are required for transporting to a place to be utilized. Further, since the water content contained therein is condensed into drains, a draining device has to be disposed at the midway of long distance pipelines, which is disadvantageous in view of maintenance.
As a result of the present inventors' earnest studies, it has been found that in a recovering-ammonia step of a wet-processing method for combustion ashes of petroleum fuels, by flowing down a gypsum slurry from the upper portion of a packed column and blowing air or stream from the lower portion thereof so as to bring air or stream into counter-current contact with the gypsum slurry, ammonia is stripped from the gypsum slurry, an ammonia stripping efficiency from the gypsum slurry in the ammonia recovery step of the wet-processing method is excellent and the clogging in the packed column is suppressed, that is, though it has been considered that the packed column can not be used for a slurry containing a large amount of solids because of a clogging problem since the packed column contains a large amount of packings and has a narrowed channel, the packed column in the wet-processing method can be used with no practical problem. On the basis of the above-mentioned finding, the present invention has been attained.